A comparator, in its basic form, compares two input signals to determine which one is of higher magnitude. It outputs a high or low response depending on whether a first input is higher than a second input. Comparators have internal sources of errors that cause the output voltage level transitions to occur at a level different than when the input voltage levels transitions pass being equal. This error is referred to as the offset voltage.
One current way of way of compensating for offset voltage is by trimming the device. In this method, a set of resistors is implanted on a chip. During the manufacturing process, but after the chip is made, the bias is determined and compensated for by fusing or severing the links between the resistors on the chip. On-chip trimming techniques can reduce offset voltage to a very low value.
The trimming technique has a drawback in that it is one time or a single shot operation and is therefore only provides compensation for the offset at the conditions under which it was tested. For a system that may undergo a wide range of operating conditions, trimming may not be optimal. This is because the actual comparator offset voltage value is subject to deviations due to variations in input common mode voltage, temperature, and supply voltage.